The present invention relates to a cryogenic storage and delivery apparatus and method in which three tanks are used to store a cryogen as a pressurized, saturated liquid, to dispense the cryogen as a vapor by vaporizing the cryogen from the bottom region of one tank and introducing the vaporized cryogen into the bottom region of another tank, thereby to produce scrubbed cryogenic vapor in the top head space region of such tank, and to form a product stream substantially free of contaminants from the scrubbed cryogenic vapor. The utilization of the three tanks takes place in an out of phase cycle designed such that the top head space region of each tank is maintained substantially clear of contaminants.
Liquid cryogen is stored in cryogenic storage facilities consisting of one or more tanks from which the cryogen is delivered from each tank upon demand. Associated with a tank is a pressure building circuit to build the pressure within the tank to a delivery pressure and a heat exchanger, attached to an outlet of the tank, to supply the product at ambient temperature.
The cryogen is delivered from the tank at a delivery pressure. Hence, when a tank is filled, it is necessary to maintain the delivery pressure. This is accomplished by alternately filling the tank from top and bottom inlets in the top and bottom regions of the tank. This creates a problem due to the fact that higher boiling contaminants such as moisture solidify on the walls of the tank and airborne dust drawn in during the filling of the tank also is deposited on the walls of the tank. Thus, after a filling of the tank, the product is initially delivered with a high concentration of the contaminants. This problem is exacerbated due to the pressure building circuit which draws off cryogen from the bottom of the tank and returns cryogenic vapor along with contaminants to the top region of the tank, the region where the cryogen is delivered. In order to solve the contaminant problem it has been known to employ either a tank within a tank or two tanks in which cryogenic vapor from one tank is bubbled through the bottom of another tank in order to solidify the contaminants prior to delivery of a cryogenic liquid or to purify cryogenic vapor prior to delivery. See for instance, U.S. Pat. No. 3,798,918 and U.S. Pat. No. 4,579,566.
Another problem involved in the storage and delivery of the cryogen is that the physical properties of the cryogen are not uniform. The reason for such non-uniformity is that the cryogen is initially delivered from a tank truck in which the cryogen is stored at a much lower pressure than the intended delivery pressure. As the tank is alternately filled from the top and bottom inlets, the cryogen when entering from the top of the tank becomes saturated while the cryogen entering from the bottom of tank subcools due to the exertion of the increased tank pressure. The end result is a top strata of saturated liquid and an underlying layer of subcooled liquid is formed. The properties of the cryogen vary with such strata and hence, the cryogen is delivered with non-uniform properties over time. The varying properties are exasperated by the continuing use of the normal pressure building methods.
As will be discussed, the present invention provides a cryogenic delivery method and apparatus for delivering a product stream composed of stable, saturated cryogen substantially free of contaminants.